Fabrication of freestanding micro hollow tubes by template-free localized electrochemical deposition

ABSTRACT

The present invention relates to a method of fabricating a micro hollow tube. More specifically, the invention relates to a method of fabricating a micro hollow tube by template-free localized electrochemical deposition, in which the micro hollow tube is fabricated by the accurate control of the distribution of the electric field strength during deposition with precise interplay of the applied voltage and the distance between the microelectrode and the grown structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating a micro hollow tube. More specifically, the invention relates to a method of fabricating a micro hollow tube by template-free localized electrochemical deposition, in which the micro hollow tube is fabricated by the accurate control of the distribution of the electric field strength during deposition with precise interplay of the applied voltage and the distance between the microelectrode and the grown structure.

2. Background of the Related Art

The fabrication of micro-devices is a fundamental issue in modern technology. Diverse techniques have been developed to fabricate microstructures consisting of semiconductors, metals and polymers. Especially, freestanding three-dimensional (3D) hollow tubes are particularly promising for broad applications in diverse areas such as optics, electronics, medical technology and microelectromechanics.

Such structures are typically fabricated by conventional lithographic process, LIGA process (Marc J. Madau, Fundamentals of Microfabrication: The Science of miniaturization (CRC press, 1997)), track-etch method (Martin C R, Van Dyke L S, Cai Z, Liang W, J. Am. Chem. Soc. 112, 8976 (1990)) and laser-assisted chemical vapor deposition (LCVD) (Lehmann O, Stuke M, Science 270, 1644(1995)).

So far, the most useful technique for producing 3D structures was the LIGA process using synchrotron x-rays that combines lithography with electrochemical metal deposition. Although quite successful, this process is affected by some significant problems: it implies multiple fabrication steps, long fabrication times, high cost due to the use of sophisticated masks or moulds; furthermore, the electroplating solution cannot easily fill high aspect ratio trenches encountered during the process. In general, LIGA finds it difficult to produce complex 3D structures.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a low cost, fast and simple method for fabricating a micro-tube with the high aspect-ratio and uniform property. To accomplish the above object, according to one aspect of the present invention, there is provided a novel method of fabricating a micro hollow tube by template-free localized electrochemical deposition, the method comprising the steps of:(a)placing a microelectrode (anode) very close to a substrate (cathode) immersed in a plating bath; and (b) applying a voltage greater than a critical voltage to the microelectrode and the substrate via a electrochemical medium, and thereby to form a micro hollow tube structure on the substrate; wherein the critical voltage is defined as the applied voltage when a maximum electric field position moves from the center of the end of the micro hollow tube structure into the edge of the end of the micro hollow tube structure just below the rim of the tip of the microelectrode.

Preferably, the method of fabricating a micro hollow tube further comprises (c) moving up the microelectrode from the formed micro hollow tube structure, with a contact growth mode being kept during deposition.

Preferably, a position of the microelectrode relative to the substrate or the micro hollow tube structure is directly observed by using an image collecting apparatus.

Preferably, the image collecting apparatus is a microradiographic apparatus with coherent X-rays in real time.

Preferably, the microradiographic apparatus comprises a X-ray beam source, a sample stage, and an image detecting means.

Preferably, the movement of the microelectrode is performed with three stepping motors in sub-microns.

Preferably, the micro hollow tube comprises at least one selected from the group consisting of a metal and a metal alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which:

FIG. 1 shows FE-SEM images of a 3D copper wire fabricated with an applied voltage of 4.5 V and with an electroplating solution of CuSO₄.5H₂O (250 g/L), H₂SO₄ (75 g/L). In (a) we see an overall picture of the wire revealing two different growth regimes, illustrated in detail in (b) and (c). Specifically, (c) is the dense growth obtained for L≈40 μm and (b) the porous one produced when L is reduced to a few micrometers. The top view in the inset of FIG. 1 b reveals a hollow-shaped feature with diameter not far from the 50 μm value of the microelectrode (dashed circle);

FIG. 2 is the distribution map of the electric field strength for different values of the applied voltage V and of the distance L. (a) V=4.5V and L=40 μm; (b) V=4.5V and L=5 μm; (c) V=10.0V and L=5 μ;

FIG. 3 is LECD growth of well-defined hollow tube. Top: FE-SEM images and (inset) tomographic slices of the grown structures; Bottom: real-time images of the LECD process by coherent x-ray microradiography. The images show: (a) a dense wire obtained at 4.5V in the no-contact growth mode; (b) a porous wire obtained at 4.5V in the contact growth mode; (c) a well-defined hollow tube obtained at 10.0V, again in the contact growth mode by the method of the present invention. The contrast difference in (c) between the inner (black arrow) and outer (white arrow) regions in the radiographic image reveals the formation of a hollow tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the invention will be hereafter described in detail, with reference to the accompanying drawings.

We developed a novel approach based on localized electrochemical deposition (LECD) with significant advantages with respect to LIGA for the fabrication of metallic micro hollow tubes. Specifically, it is a simple, inexpensive, and damage free method.

The LECD approach is based on electrochemical deposition: the microelectrode (anode) is placed very close to the conducting substrate (cathode) immersed in the plating bath. As the voltage is applied and the microelectrode is moved up, a metallic microstructure is fabricated that protrudes towards the microelectrode. The process is thus particularly suitable for producing high-aspect-ratio metallic structures with a variety of features. This simple approach can be applied to different materials such as metals, metal alloys, conducting polymers and semiconductors to fabricate objects in the micrometer, sub-micrometer, and nanometer scale.

We conducted experiments at room temperature using 1.05M CuSO₄.5H₂O, 0.8M H₂SO₄. The microelectrode with 50 μm in diameter was prepared by sealing Pt wire (99.95%, Alfa Aesar) in a glass tube and then by polishing the surface. Platinum coated silicon wafers were used as cathodes. The microelectrode position was accurately controlled by three stepping motors. The experiments were performed at the “7B2 X-ray Microscopy” beamline of the Pohang Light Source (PLS), Korea. Additional tests such as field emission scanning electron microscopy (FE-SEM, JEOL JSM6330F) were also used to study the microscopic characteristics of the grown structures. The microradiographic monitoring of the LECD process was implemented in situ in a specially designed miniature electrochemical cell machined from a Teflon block and sealed by Kapton films that were x-ray transparent and stable for most chemical reactions. The distance between the two cell windows was optimized to ≈5 mm to avoid unnecessary x-ray absorption by the plating electrolyte. For the micro-tomography, the grown structure was mounted on a translation/rotation stage with precise positioning (250 nm/0.002°) and one thousand projection radiographs were taken while rotating the sample between 0° and 180°. The slice images of the grown structure were then reconstructed by using a self-developed reconstruction algorithm.

FIG. 1 shows FE-SEM (field-emission scanning electron microscope) images of a 3D copper wire fabricated by the LECD process with an applied voltage of 4.5 V. FIG. 1( a) shows that the wire so produced reflects two growth regimes: the upper part [shown in detail in FIG. 1( b)] corresponds to a regime yielding a porous microstructure, whereas the other regime results in a dense uniform microstructure [FIG. 1 (c)]. The dense uniform growth was obtained with a relatively large distance between the microelectrode and the growing structure, L=40 μm (no-contact growth mode). The dense uniform growth abruptly changed to a porous growth when L was reduced to a few micrometers, (contact growth mode).

In order to understand this change in the growth characteristics, we must consider the mass transport mechanisms of metal ions. Diffusion of metal ions from the bulk solution to the cathode dominates conventional electrochemical deposition; in LECD, however, we must take into account the migration of metal ions that is driven by strong localized electric fields. The distance L determines the interplay between diffusion and migration. Specifically, diffusion prevails at large L-values whereas migration increasingly dominates as L decreases. When L reaches the critical value at which migration replaces diffusion as the dominating factor, the deposition rate rapidly increases because of the strong electric fields, changing the grown structure from dense to porous as seen in FIG. 1. The top view shown in the inset of FIG. 1( b) reveals a hollow feature within the porous wire; the diameter of this feature is not far from that of the microelectrode (dashed line).

One of the factors that affect the growth characteristics is the electric field strength distribution near the grown feature. We modeled this distribution and the results are illustrated in FIG. 2. For a low applied voltage of 4.5V and a large L-distance of 40 μm, the electric field strength exhibits a maximum value at the center of the grown feature [FIG. 2( a)]. We expect, therefore, the formation of a wire with a cone on top. As L decreases to 5 μm—a value much lower than the critical level [FIG. 2( b)]—the maximum field position moves to the edge of the grown structure just below the rim of the microelectrode. Thus, the formation of the porous region with the hollow feature of FIG. 1 (b) can be explained by the electric field edge enhancement at the microelectrode rim that induces a high migration rate below the rim.

As the applied voltage increases from 4.5 to 10V for L=5 μm, the electric field strength sharply increases at the microelectrode rim, enhancing the field strength difference with respect to the microelectrode core—see FIGS. 2( b) and 2(c). Consequently, the electrochemical deposition is also enhanced below the rim. The consistent results of the field simulation and of the actual growth thus suggest that it is possible to change the copper grown structure from a dense wire to a well-defined hollow shape simply by controlling the electric field distribution near the microelectrode.

These findings lead us to suitable strategy to fabricate well-defined hollow tubes. The strategy is based on the control of the electric field strength near the grown structure as suggested by FIG. 2: as the applied voltage is increased in the contact growth mode (the migration dominant regime), the enhancement of the growth below the microelectrode rim eventually produces a tube rather than a wire.

FIG. 3 is the results of this approach for the production of copper grown structures on Pt substrates. The top of FIG. 3 shows FE-SEM images while the bottom demonstrates microradiographic images obtained by real-time coherent x-ray imaging. A dense wire is produced at 4.5V by the no-contact growth mode [FIG. 3( a)]. The tomographic slice reconstruction in the inset of FIG. 3( a) shows that the wire is not only dense but also uniform. The cone shape on top of the wire is the result of the field-induced local migration discussed above—see FIG. 2( a).

On the other hand a porous structure is obtained at 4.5V in the contact growth mode as illustrated in FIG. 3( b)—but a dense rim feature is present around the porous structure (white arrow), as confirmed by the x-ray tomographic slice in the inset of FIG. 3( b, top). As the applied voltage is increased to 10V, the grown structure changes to a well-defined hollow tube with a very uniform wall thickness (≈5 μm)—as shown by the FE-SEM image of FIG. 3( c, top) and by the corresponding tomographic slice in the inset. This is the limit result of the migration enhancement near the rim produced by a highly confined, strong electric field. The coherent x-ray micro images of FIG. 3( c, bottom) illustrate this process in real time.

In conclusion, we demonstrated that a careful manipulation of the electric field strength distribution and in general of the growth parameters enables LECD to fabricate well-defined metallic micro hollow tubes. These results were made possible by a careful control of the interplay between migration and diffusion, in turn determined by the field strength. The practical cases discussed here are only a few examples of the broad variety of metal structures that our novel LECD approach can produce by appropriate tuning of the growth parameters.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention. 

1. A method of fabricating a micro hollow tube by template-free localized electrochemical deposition, the method comprising the steps of: (a) placing a microelectrode (anode) very close to a substrate (cathode) immersed in a plating bath; and (b) applying a voltage greater than a critical voltage to the microelectrode and the substrate via a electrochemical medium, and thereby to form a micro hollow tube structure on the substrate, wherein the critical voltage is defined as the applied voltage when a maximum electric field position moves from the center of the end of the micro hollow tube structure into the edge of the end of the micro hollow tube structure just below the rim of the tip of the microelectrode.
 2. The method of fabricating a micro hollow tube according to claim 1 further comprises (c) moving up the microelectrode from the formed micro hollow tube structure, with a contact growth mode being kept during deposition.
 3. The method of fabricating a micro hollow tube according to claims 1 and 2, wherein a position of the microelectrode relative to the substrate or the micro hollow tube structure is directly observed by using an image collecting apparatus.
 4. The method of fabricating a micro hollow tube according to claim 3, wherein the image collecting apparatus is a microradiographic apparatus with coherent X-rays in real time.
 5. The method of fabricating a micro hollow tube according to claim 4, wherein the microradiographic apparatus comprises a X-ray beam source, a sample stage, and an image detecting means.
 6. The method of fabricating a micro hollow tube according to claim 2, wherein the movement of the microelectrode is performed with three stepping motors in sub-microns.
 7. The method of fabricating a micro hollow tube according to claim 1, wherein the micro hollow tube comprises at least one selected from the group consisting of a metal and a metal alloy. 